(a) Field
The subject matter disclosed generally relates to guidewires for catheter use. More particularly, it relates to pressure guidewire technology.
(b) Related Prior Art
Conventional Guidewires
Conventional guidewires are made using solid rods. They may be made using a combination of different materials such as hard tempered stainless steel and nitinol. Stainless steel provides good pushability and torquability, and is relatively cheap, while the nitinol provides good yield strength, i.e., does not permanently deform. Nitinol is nickel/titanium alloy having superelastic properties as known by those skilled in the art. One consideration in designing guidewires is that welding nitinol to stainless steel is extremely challenging.
Typical guidewires comprise three main parts as described below.
The first part is the proximal section. For coronary use, the proximal section, i.e., the length prior to the last 25 to 40 cm, is made typically of 0.014″ outside diameter (OD) stainless steel. Nitinol is not preferred in this section because it does not provide a good pushability and torquability and it is fairly expensive. This section always resides in the catheter guide, hence not directly into the blood vessel. In use, a portion of this section is outside the patient's body and is controlled by the person using the guidewire. The typical length of the proximal section is 140 cm or longer.
The second part is the middle section. The middle section is the following 25 to 40 cm of flexible wire, before the tip portion, i.e., the third portion extending further over another 3 to 4 cm. The middle section is the section that navigates within the coronary arteries. Guidewires may use a nitinol wire of about 0.0095″ OD over this section. In order to improve the navigability within the vessel, i.e., having the ability to be threaded through sharp corners within vessels, the distal 1 to 3 cm portion of the middle section, may be slightly tapered, the outside diameter being reduced roughly from 0.0095″ to 0.007″. This taper allows the guidewire to bend with a circular shape, as opposed to a parabolic shape in the case where the outside diameter is uniform. This section can be bent quite seriously when navigating into the vessels and hence, shape retaining is a desirable feature, although not a necessary one.
The third part is the tip section. The tip is the last 3 to 4 cm of the guidewire. It is made with a very thin tapered, or shaped otherwise, spring tempered or hardened stainless steel core wire. The core wire is surrounded by tungsten, platinum, palladium or another wire winding providing smooth radiopaque tip. Tip section can be pre-shaped prior to use by physician to ease the navigation in any given circumstance.
Guidewire features are:                Good trackability: Guidewire can be threaded through tortuous vessels;        Good pushability: Guidewire can be pushed through tortuous vessels and through stenosis;        Good torque: Guidewire can be smoothly rotated to orient the tip within the vessel;        Soft tip: Guidewire is atraumatic for vessels; and        Good support: A balloon catheter can be slid over the guidewire without the guidewire prolapsing or kinking.        
Pressure Guidewire
A pressure guidewire is one that includes a tip pressure sensor. Trade-offs are necessary to route the sensor lead wire(s) through the guidewire, resulting in sub-optimal mechanical design. Existing pressure guidewires typically comprises four main parts as described below.
The first part is the proximal section. The proximal section is made with a stainless steel hypotube (aka the proximal tube) having typical OD of 0.014″. There are three sensor lead wires that are routed through the proximal tube to connect to tip sensor.
The second part is the middle section. A solid core is welded to the proximal section on one end, and to a sensor housing on the other end. This solid rod is typically 27 cm long. The three wires of typical piezo-resistive sensor run through the proximal tube and on the wall of the core along the 27 cm. The three wires are finally directed within the sensor housing (described below) to be connected to the sensor.
The solid core and three wire assembly are protected either by providing a polyimide tube covering the assembly, by providing a spiral wound wire or other method for the assembly. The foregoing covering method provides minimal mechanical benefit and therefore they are used mostly as a protective sheath.
The third part of the pressure guidewire is the sensor housing. The sensor housing is made with a hypotube with internal diameter (ID) accommodating the tip sensor and an OD that is the continuity of the proximal section OD. An opening is provided to the sensor housing to let blood communicate with tip sensor.
The fourth part of the pressure guidewire is the tip section. The tip section is provided at the end of the sensor housing. A conventional tip section can be used as there is no wire to be passed therethrough.
The critical part of above described pressure guidewires is the middle section (second part above). Wall thickness of polyimide or wire winding added to the size of the sensor lead wires that runs along the solid core compromise the use of optimal solid core. More specifically, the solid core is usually too thin for delivering adequate pushability and torquability on one hand, and adequate support for a balloon catheter on another hand.